Your question can have two different answers…
Do you want to have attendees only have the Aurthorize.net option for all your events?
If so, you do not need this add-on. If you are using one single gateway for all your events, make sure that that one (Authorize.net, in your case)is the only activated one by simply going to Dashboard -> Events -> Payment Gateways and deactivate “Offline” using the inline action, shown on mouse-over.
https://paste.pics/e2b19b1caa4f0bc801691dfbbfe7e96f
Even if the Offline gateway is deactivated in these settings, an Admin can still add a manual booking (automatically using the Offline Gateway) here:
https://paste.pics/283f1c959e6ba2e894f546bea5d4ffb9
If you meant that you want to disable the Offline Gateway only for specific events (requiring this add-on), please share a screenshot of the gateway selection section in that Edit Event Page and a live link to the front-end version of that page.